1. Field of the Invention
The present invention concerns a strapping machine and, more particularly, a device to control the feeding of the strap, allowing a more precise and rapid control of the strap stopping or speed-changing function.
As known, machines of this type comprise the drive means which draw the strap from a reel, launch it into a special guide track around the product to be packed, and then recover it--after its leading end has been blocked or clamped--so as to wind it around the product with a reset tension. Further means then provide to tie and cut the strap after its windup.
It is also known that modern packing technique requires, on one hand, short operating times and thus high strap launching and recovery speeds and, on the other hand, the possibility to regulate the tensioning of the strap to a wide extent. However, there two requirements are often scarely compatible.
2. Description of the Prior Art
A strapping machine which tries to satisfy both these requirements is described, for example, in the Italian Patent No. 1,135,722, filed on Mar. 24, 1981, by the same Applicant, to which reference is made herein for a better understanding of the present invention. This machine has turned out to be more than satisfactory for many years, but it is no longer apt to fully satisfy the present requirements of packing speed.
One of the problems arising with the increase of the strap feeding speed is its instant and prompt stopping at the end of the launching stroke and, respectively, at the end of the recovery stroke.
As known, the strap is moved forward between a driving wheel and an idle pressure wheel. During launching, when the leading end of the strap reaches the stop, it is blocked in correspondence of the clamping gripper where it operates a microswitch which controls the stopping of the drive motor. Although there are known to be several arrangements which try to guarantee an instant stopping, in practice, the inertia of the rotating masses of the driving unit causes--at the high speeds currently requested--irregular behaviours. In particular, as a result of its residual drag, the strap tends to curl up or form a loop along its path between the driving wheel and its blocked leading end.
According to the teaching of FR-A-2,621,555, or of U.S. Pat. No. 3,949,662, said loop formed by the strap is used to cause the lifting of an oscillating guide, which partly surrounds the driving wheel and which in turn operates a switch to stop the main drive motor.
The actual advantage of these known arrangements is that the strap does not wedge into the feeding path and does not hence create any jamming, in that it freely extends forming said loop. Nevertheless, said arrangements provide no advantages as far as the prompt, instant, stopping of the strap feeding stroke, since it is evident that the switch operated by the oscillating guide--when this latter is moved by the loop being formed by the strap--anyhow operates at a moment which is subsequent to that of operation of the microswitch controlled by the leading end of the strap reaching the clamping gripper.
Furthermore, both of the arrangements known from the aforecited patents merely operate during launching of the strap.
A similar problem arises however during the fast recovery of the strap: in fact, the residual drag causes an undesired tightening of the strap around the product to be packed, at a tension higher than that expected, which can often cause damage to the product being packed.